ltcc-based sensors for mechanical quantities papers/midem_42(2012)4p260.pdf · 262 figure 4 shows...

260

Original scientific paper

MIDEM Society

Journal of Microelectronics Electronic Components and MaterialsVol 42 No 4 (2012) 260 ndash 271

LTCC-Based Sensors for Mechanical QuantitiesUwe Partsch1) Christian Lenz1) Steffen Ziesche1) Carolin Lohrberg1) Holger Neubert2) Thomas Maeder3)

1) Fraunhofer IKTS Dresden Germany 2) Technische Universitaumlt Dresden Germany 3) Laboratoire de Production Microtechnique EPFL Lausanne Switzerland

Abstract Besides their excellent dielectric and thermo-mechanical characteristics Low Temperature Cofiring Ceramics (LTCC) are also well suited for the fabrication of 3D micromechanical components such as sensors for mechanical quantities This paper describes the development of such sensors covering some material and technological aspects Furthermore the design process for mechanical sensors is discussed as well as application examples of sensors for the detection of pressure force acceleration and flow

Key words LTCC Sensors 3D-Integration

Senzorji mehanskih veličin na osnovi LTCCPovzetek Poleg odličnih dielektričnih in termo-mehanskih lastnosti so keramike z nizko temperaturo žganja (LTCC) primerne tudi za izdelavo 3D mikro mehanskih komponent kot so senzorji mehanskih veličin Članek opisuje razvoj teh senzorjev vključno z nekaterimi tehnološkimi in materialnimi vidiki Dodatno je predstavljen postopek oblikovanja mehaničnih senzorjev in nekaj primerov senzorjev tlaka sile pospeška in pretoka

Ključne besede LTCC senzorji 3D-integracija

Corresponding Authorrsquos e-mail uwepartschiktsfraunhoferde

1 Introduction

Ceramic sensors are applied when specific require-ments have to be fulfilled eg a high reliability at el-evated and cycled temperatures harsh environment as well as in aggressive chemicals

The ceramic multilayer technology (eg LTCC) enhances these advantages because of its ability (i) for a complex 3D miniaturization with embedded deformable bodies (cantilever diaphragms) channels and cavities as well as the ability (ii) for the direct integration of electronic components for signal conditioning and processing

One reason for the outstanding commercialization suc-cess of LTCC-based sensors is the cost level which is mainly defined by material and process costs In order to reduce material and process costs miniaturization is the most important leverage

2 Material aspects

The manufacturing of LTCC-based 3D micro-compo-nents means the co- and post-firing of glass-ceramic

layers and different functional materials During co-firing in particular intensive mechanical and chemical interactions can appear strongly influencing the com-ponent performance

The successful processing of multilayered multi-mate-rial based miniaturized LTCC components requires the proper control of different materials and technological aspects

21 LTCC

One reason that ceramic materials are advantageously used for deforming bodies in sensor applications is their linear stress vs strain behaviour Table 1 compares different ceramic materials in terms of their mechanical properties

261

U Partsch et al Informacije Midem Vol 42 No 4 (2012) 260 ndash 271

Table 1 Mechanical properties of different ceramics (CTE hellip coefficient of thermal expansion (20 400degC) E hellipYoungrsquos modulus sB hellip bending strength)

Material CTE E sB sBE

ppm K GPa MPa 10-3

96 Al2O3 1) 76 350 310 09

998 Al2O3 2) 75 406 630 16

LTCC 3) 58 120 320 27YSZ 4) 112 210 1050 50ZTA 5) 81 357 1350 38

Datasheet values 1) CeramTec V38 2) CeramTec RK 87 3)

Du Pont DP 951 4) CeramTec MZ 429 5) CeramTec DC 25

The sBE ratio determines the dimension of the deform-ing bodies in terms of sensitivity and overload stabil-ity The larger the sBE ratio the smaller the deforming bodies can be designed It can be seen that LTCC is well suited because of a sBE ratio of 27 However YSZ (sBE = 5) and ZTA (sBE = 38) have a better mechanical per-formance but embedding eg of low sintering noble metals as well as resistors can only be realized using a low temperature firing system

Different types of LTCC show a different bending strength behavior [1] Best values can be obtained us-ing the Du Pontrsquos Green Tape 951 system (Figure 1) It must however be noted that in order to ensure long-term reliability static [2] and cyclic fatigue [3] must also be accounted for

Figure 1 Comparison of different LTCC types regard-ing their fractural strength (re-calculated after [1])

The 951 system offers different tape thicknesses (50 ndash 254 microm unfired) as well as a full system of the required pastes (innerouter conductors via outerinner resis-tors) This makes it particularly well suited for the fab-rication of sensors

The shrinkage control of LTCC multilayer components plays an important role regarding the final dimension-al control eg for advanced electronic packages (chip sized packages flip chip) but also in the case of me-chanical sensors

Shrinkage occurring during firing can be influenced by controlling the process factors (of lamination and fir-ing) Figure 2 shows the influence of lamination tem-perature and pressure

Figure 2 LTCC DP 951 X-Y shrinkage (SX-Y) vs lamina-tion temperature (T) and pressure (p) [4]

22 Functional Thick Films

221 Metallization

In many cases LTCC-suited inner metallization pastes are silver-based The interaction between silver and LTCC during co-firing was described in the past by sev-eral authors eg [4-6] it can lead to significant warping of thin and 3D structured LTCC geometries

Figure 4 LTCC DP 951 X-Y shrinkage (SX-Y) vs heat up ramp (HR) and metallization degree (MD) [4]

σ

262

Figure 4 shows that the shrinkage of a DP 951-based LTCC multilayer during firing is ndash in addition to the lamination parameters ndash also influenced by the degree of metallization and the heat up ramp It can be seen that there is only a slight effect on the X-Y shrinkage by changing the heat up rate when the multilayer is not metalized In the case of an extensive metallization (60 metallization area) there is a strong dependence on the heat up ramp The metalized multilayer shrinks in the same way as the non-metalized multilayer at higher heat up ramps (128 8Kmin) Using low heat up ramps (2 Kmin) the shrinkage is reduced to 121 The silver layer has a ldquolockingrdquo function due to the different shrinkage behavior

Besides this pure mechanical effect there are also chemical interactions between the silver layer and the LTCC [6] and [5] demonstrated a strong degree of sil-ver diffusion into the LTCC which can reach some 10 microm [7] showed that silver influences the viscosity of the LTCC glass leading to an earlier densification and crystallization Because silver enters the LTCC glass ma-trix in an oxidized status a nitrogen sintering atmos-phere can minimize the discussed effects by shifting the chemical equilibrium between Ag+(glass) and Ag0 (metal) in favour of the latter

222 Piezo-Resistors

The transformation of a mechanical quantity into an electrical signal by piezo-resistors is one of the most common measuring principles Using a deformable body with piezo-resistors in the areas with high me-chanical strain a mechanical input quantity (eg force) can be transduced into a proportional resistance bridge voltage change (Figure 5)

Figure 5 Sensor transmission chain [8]

Based on their microstructure and effective conduct-ing mechanism thick-film resistors (TFRs) always have a strain sensitive behavior

As well as the thermal behavior the strain sensitivity of TFRs is dominated by insulatingsemi-conductive nanoscale glass layers between conducting particles within the three-dimensional conductive chains strain-dependent interparticle tunneling being the favoured mechanism [9 10]

The strain sensitivity is specified using the K- or gauge factor which determines the ratio between the relative change of resistance and the applied strain

Measured gauge factors of TFRs are between 2 and 35 They are influenced by (i) the composition of the TFR (type grain sizes and proportion of glasses and con-ductive phases) (ii) the firing process (dissolution of the conductive phase into the glass) and (iii) the inter-actions between the TFR and the substrate as well as the terminations (diffusion of silver ions into the TFR glass matrix) [11] The maximum signal gain of TFRs is limited by noise Therefore the effective signal-to-noise ratio is suitable as a normalized parameter [12]

(1)

US and UN are the effective signal voltage and the peak-to-peak noise voltage Here US was calculated using the measured gauge factors a supply voltage of 5 V and a relative strain of e = 2710-4 (corresponds to 50 of the maximum deflection before substrate breakage) UN was calculated by solving the definition of the noise index (NI)

Figure 6 Strain sensor paste development [12]Noise index (NI) vs BET surface area of RuO2

In order to improve the SNReff [12] compositions were developed as a mixture from RuO2 (four different par-ticle size distributions one mixture) as well as two dif-ferent glasses (five different particle sizes) The aim of these investigations was a TFR sheet resistance of 10 kOhmsqr

The results showed that the noise index (NI) and the gauge factor of TFRs are strongly influenced by the par-ticle size distribution of the glasses as well as the con-ductive phase Figures 6 and 7 illustrate that there is a

Piezo-resistor

Wheat-stone

bridge

Deform-ablebody

Force F Stressσ

Strainε

ΔR (ε)

Bridge voltage UB

Displacementw

Mech - Elec - Transducer

UBUV = KεR = R0(1+Kε)


263

compromise between gauge factor and noise behavior of TFRs

Figure 7 Strain sensor paste development [12] Longi-tudinal gauge factor vs BET surface area of RuO2

However these developments cannot improve the DP 2041 characteristics regarding the SNIeff

Further activities are planned concerning the devel-opment of new glass types the optimization of firing and the analysis of alternative conductive phases (py-rochlores)

Figure 8 Effective signal-to-noise ratio (SNReff ) [12] Comparison of IKTS and commercial pastes

223 Sacrificial Materials

Sacrificial materials temporarily stabilize functional elements during lamination and sintering They are re-moved during or after sintering Two types of sacrificial materials are differentiated- Mineral sacrificial paste (MSP) - Fugitive sacrificial paste (FSP)

MSP consists of glass an active substance and vehicle [13 14] The active substance is a mineral filling mate-rial After sintering the MSP provides a solid layer and can be removed with aqueous acetic acid MSP suits well for cantilevers and freestanding structures with high level requirements regarding the accuracy Closed structures are difficult to handle as MSP has to be dis-solved with acid FSP consists of organic and graphite powder and con-tains no glass phases [15 16] The graphite is oxidized to carbon dioxide at about 700 degC Thus it is well suited for closed cavities channels and diaphragms in LTCC because FSP volatilizes through the porous LTCC or open channels during sintering [15]

In order to stabilize embedded mechanical structures during sintering FSP oxidation must be prevented us-ing a non-oxidizing sintering atmosphere (eg nitro-gen) In this case a ldquoventrdquo is required for out gassing the carbon dioxide from the embedded structures The appropriate timing for the change from oxygen to ni-trogen and vice versa is a critical issue It was studied for DP 951 in [17]

The resulting profile is shown in Figure 9 The amount of FSP and the geometry of the embedded structures require an adjustment of the sintering procedure for every component to avoid warping and FSP residue ef-fects

Figure 9 Customized sintering profile for surviving FSP after sintering DP 951 [17]


264

However sacrificial materials often require the applica-tion of zero-shrinkage techniques [18] because of their unadapted shrinkage behavior in comparison to stand-ard LTCC materials

3 Technological aspects

The higher functional integration in multilayer-based sensors and microsystems complicates their process-ing compared to that of multilayer substrates for elec-tronic assemblies Sensors for mechanical quantities in particular need (i) functional structures like free-standing deforming bodies fluidic channels or heating bridges which have to be processed with no sagging or warping (ii) deforming bodies with reinforced regions for seismic masses or force application and (iii) low di-mensional tolerances of the processed substrates to provide low variances of the performance of the fab-ricated sensor elements It is a challenge to customize the standard LTCC technology for these aspects How-ever the technology varies according to design mate-rial and dimension of the sensor elements

31 Sagging and warping

Sagging and warping are mainly caused by the lamina-tion and sintering process Below some techniques are mentioned in order to reduce these imperfections- Cold lamination (uniaxial) at 30 degC reduces sag-

ging- An isostatic pre-lamination of the unstructured

tapes provides higher stability for the following processes like lamination However higher pres-sure ranges will then be necessary in the follow-ing lamination steps

- A differential shrinkage of several single layers can be achieved by pre-laminating which helps to minimize sagging effects at thin deforming elements (diaphragms cantilevers) during firing (Figure 10)

- Fixation and support of freestanding structures during sintering reduces sagging Outer struc-tures can be directly supported on the sintering support Embedded structures have to be stabi-

lized using additional layers like MSP or FSP Cus-tomized sintering profiles should then be applied

- Customized sintering profiles for integrated thick-film metallization reduce warping

32 Structural elements for reinforcement

Some devices require deforming bodies with rein-forced structures to supply flexural resistant proper-ties (eg accelerometers or force sensors) For a precise and reproducible fabrication special techniques were established to process these bodies in multilayer tech-nology- Little bars fix the reinforced regions to the outer

frame during stacking and laminating (Figure 11) After laminating or sintering these bars have to be removed by laser cutting However it must be considered that no other functional structures are placed under the bars which might be dam-aged by the laser

- Reinforcing layers eg reinforced centers of diaphragms can also be stacked separately as shown in Figure 11 However this technique re-quires a customized stacking tool and the process gets more complex

a bFigure 11 Techniques for reinforced regions a) Fixing bars which are removed after laminating or sintering b) separate stacking

33 Dimensional tolerances

Figure 12 Different shrinkage of a DP 951 4-inch sub-strate - interpolation between 25 data pointsFigure 10 LTCC diaphragm Different pre-lamination

steps can minimize sagging effects


265

Design miniaturization together with panel production of LTCC microsystems offers low-cost applications This is one of the advantages of the LTCC technology

However small dimensional tolerances have to be met in order to provide reproducible properties of the sen-sors and to achieve an acceptable yield- Low tolerances have to be ensured for the struc-

turing processes (punching laser structuring screen printing and stacking) over the whole working space of a multiple printed panel

- The lamination process has to be controlled (eg plan parallelism at the uniaxial lamination press) to ensure homogeneous results

- The nominal shrinkage varies over the fired sub-strate up to 05 (Figure 12) It depends on the lamination process the structures [19] and the sintering process For best results the variation of the shrinkage over the working space of the pan-el has already to be considered in the CAD files during the design phase

4 Sensor design

Even though the LTCC technology provides many dif-ferent applications for mechanical sensors the general design process is almost the same (Figure 13)

Figure 13 General design process for mechanical sen-sors in LTCC

At the beginning technical specifications have to be defined (eg sensor properties dimensions and bound-ary conditions)

Following that the conceptual design must be speci-fied regarding producibility of structures and useability of materials After evaluation and selection the most suitable conceptual design has to be designed in detail

Today model-based design and design optimization is inevitable when designing integrated sensors for mechanical quantities Therefore physical models of adequate levels of abstraction (granularity) have to be provided for the conceptual design and the final de-sign step as well

Afterwards the final design has to be checked and re-vised if necessary regarding the required properties For this purpose an evaluation step follows which in-cludes fabrication and characterization of prototypes In case of unsatisfying properties one or more devel-opment loops have to be passed to adapt sensor con-cept and design and to validate or improve the simula-tion models as well

Technology-inherent distributions of dimensions ma-terial properties and process parameters lead to per-formance variations of a set of sensors even when they all are of the same design and fabrication processes Performance distributions can be minimized by a fur-ther design step which involves the probability distri-butions of the design and process parameters

The objective of a so-called robust design optimizia-tion is to find a design that fulfills the target require-ments specified with minimized scattering of the sen-sor performance [20 21] As a result an improved yield is to be expected

5 Applications

51 Pressure Sensor

Integrated LTCC-based pressure sensors have many advantages in comparison to classic steel or ceramic-based pressure sensors [22-24] because of the technol-ogy-inherent features

The LTCC technology enables easily variable sensor geometries eg different diaphragm thicknesses for different pressure ranges by using different tape thick-nesses All types of pressure sensors (relative absolute differential) can be built up Furthermore all compo-nents of the sensor system (sensor body pressure con-


266

nectorsmicro piping and electronic components) can be integrated in the LTCC-based multilayer component

Figure 14 Finite elements analyses of different con-structional designs (numbers of fixation cantilever) perfect mechanical decoupling of the sensor cell with one or two fixation cantilevers

In [23] a new LTCC-based pressure sensor concept was presented aiming the improved mechanical de-coupling of the diaphragm The sensor cell is fixed by thin LTCC cantilevers containing micro channels for the pressure connection of the sensor cell

Using the piezo-resistive measuring principle the strain caused by diaphragm deflection is measured by thick-film resistors connected to a Wheatstone bridge and screen printed at the surface of the LTCC diaphragm

FE analyses were carried out for the optimization of the design of the LTCC fixation cantilever [Figure 4]

Figure 15 Characteristic curves of different sensors (pressure ranges) Shift of bridge voltage (DUb) vs ap-plied pressure (p)

The sensor characterization showed that all sensors have a strongly linear behavior (bridge voltage vs pres-sure) and are nearly free of hysteresis (Figure 15)

Table 2 shows the overall characteristics of the sensors for different pressure ranges (02 15 5 bar nominal pressure) The operation pressure of the different sen-sors was defined by additional burst tests All sensors have a 4-times overload safety

Table 2 Characteristics of different sensor types accord-ing to DINISO 16086 (offset voltage lt 25 mVV bridge resistance ~ 25 kOhm T=25 oC) D ndash diaphragm diame-ter d ndash diaphragm thickness pop ndash operation pressure S ndash sensitivity L ndash linearity H ndash hysteresis FS ndash full scale

Diaphragm p op S L H

D [mm] d [microm] [bar] [mVVtimesbar]

[FS] [FS]

45 220 50 04 006 00545 150 15 11 004 00145 45 02 46 007 008

Finally sensors with integrated signal conditioning electronics (amplifier temperature compensation) were fabricated (Figure 16)

Figure 16 LTCC pressure sensor with integrated elec-tronics for signal condition

52 Force Sensor

Force sensors always detect the effect (deformation stress strain) of an applied force on a deformable body The deformation is converted into an electrical signal by transducers working on the principle of eg capaci-tive piezoelectric or piezo-resistive measurements Though the piezo-resistive principle is best suited because of its high accuracy long-term stability and application range When this transducer principle is ap-plied cantilevers and diaphragms obtain the best per-formance for small nominal loads

∆U Partsch et al Informacije Midem Vol 42 No 4 (2012) 260 ndash 271

267

Figure 17 The cartwheel structure [8] C - cantilever Z - flexural resistant center P ndash piezo-resistors F - frame

Miniaturized piezo-resistive force sensors for tensile and compressive loads (2 N 5 N and 10 N) were de-veloped Compared to the pressure sensor above the uniform application of the force to be measured in the deforming sensing body is the main challenge of this study For this purpose a cartwheel structure was de-signed (Figure 17) It consists of four identical cantile-vers which are combined by a flexural resistant center A metal pin with thread is mounted axially in the center hole This structure obtains a uniform distribution of the applied tensile or compressive forces on the canti-levers avoids angle errors and combines both the high sensitivity of cantilevers and the robustness against shear forces of diaphragms

Four piezo-resistors are placed on the cantilevers in ar-eas of maximum strain ndash two under expansion and two under compression They are connected to a Wheat-stone bridge thus the bridge voltage composes the sensing signal

Figure 18 LTCC force sensors for three nominal loads (2 N 5 N 10 N) Dimensions 14 x 14 mmsup2

An analytical model [8] was developed for the dimen-sioning and simulation of the sensor elements The

most important restrictions for the design process were (i) to achieve a high grade of miniaturization (ii) to provide robustness against overloads up to 200 of the nominal load and (iii) an equal layout for the men-tioned force ranges except the top layer (Figure 18)

Table 3 Measured sensor characteristics (T = 25degC) F B ndash breaking load S ndash sensitivity L ndash linearity TC-S ndash tem-perature coefficient sensitivity FS ndash full scale

Nominal load

F N

FB

[N]S

[mV( VN)]L

[FS]TC-S

[SK]

2 N 4 26 lt 06 0025 N gt 10 06 lt 04 003

10 N gt 20 01 lt 10 002

The designed sensors were fabricated in multiple pro-cessing with 25 elements per 4-inch substrate The techniques pre-lamination of all sheets uniaxial cold lamination at 30degC and the fixation of the deformable bodies on the sinter support were used to minimize sagging

The measured characteristics in Table 3 show the po-tential of the LTCC as base material for this application The sensors have a linear behavior and a low tempera-ture drift of the sensitivity Furthermore there is a good compromise between sensitivity and breaking load

Figure 19 Characteristic curves of different sensors (load ranges) Shift of bridge voltage (DUb) vs applied force (F)

53 Acceleration Sensor

The measurement of accelerations requires a mass element on a spring The mass elongates and deforms the spring in response to an applied acceleration The deformation can be measured eg by a piezo-resistive transducer

∆


268

Todayrsquos acceleration sensors made of silicon offer suf-ficient functionality in a cost-effective way In contrast acceleration sensors made of LTCC work under elevat-ed temperatures or as an additional feature integrated in LTCC substrates or electronic assemblies

The LTCC multilayer technology suggests a sensor lay-out with leaf springs in a layer stressed for bending or for torsion Thus an acceleration perpendicular to the layer effects a deformation of the springs which can be measured by piezo-resistive thick-film resistors printed on the springs The layout has to consider some con-flicting requirements eg high resonance frequency ie high stiffness versus high sensitivity ie lower stiff-ness or uniform strain in the benders for a high reliabil-ity versus high strained areas of the springs for a high sensitivity

Analytical and finite element models were used to optimize the sensor design Two parallel trapezoidal benders have advantages as compared to rectangular benders or torsion springs or the bridge and cantilever structures proposed in [25] A FEA model of the opti-mized layout fabricated in LTCC is shown in Figure 20

Figure 20 FEA model of an acceleration sensor with two trapezoidal benders (equivalent strain)

The first resonance frequency has a strong influence to the working range and is obtained from FEA models A linear relation between acceleration and deformation can be expected about 50 of the resonant frequency Resonant excitation would destroy the sensor Four thick-film resistors two on the benders and two on the frame are connected to a Wheatstone bridge like it has been done in case of the pressure sensors

Figure 21 LTCC acceleration sensor with piezo-resis-tors on rectangle leaf springs

Figure 22 LTCC acceleration sensor characteristic curve bridge voltage (Ub) vs acceleration Different ex-citation frequencies

Figure 21 shows a LTCC acceleration sensor with a reso-nance frequency of 250 Hz [20] Excited with 100 Hz the sensor gives a slightly higher signal due to the closer proximity to the resonance frequency as compared to 50 or 75 Hz (Figure 22)

However the tested sensors are sensitive for large accelerations Smaller effective ranges or can be de-signed

54 Flow Sensor

Most of the commercial flow sensors work on the heat-ing wire principle A thin platinum wire which is heated by a current is placed in a flowing gas It cools the plati-num wire which refers to its resistance and influences the connected Wheatstone bridge The implementa-tion of a channel system with integrated freestand-ing heating wire in LTCC is only possible when using a sacrificial material A thin heater can be realized by overprinting the FSP with a thick-film platinum paste It enables low power loss and raises the sensitivity

∆


269

In order to reach an adjusted shrinkage between LTCC and FSP a zero-shrinkage technique is required For this purpose several techniques were investigated- The combination of DP 951 with Release-tapes

(Ceramtape A GT 951 RT) indicates delamination and residues at removal respectively

- The self-constrained LTCC-tape HL2000 (Heraeus) is not suitable to uniaxial lamination process (voids and delamination) which is required and the FSP-paste (residues after firing)

- Best results were achieved with a combination of DP 951 and HL2000 (Figure 23) DP 951 suits to the FSP-paste and stabilizes the HL2000 during the lamination process The zero shrinkage of the whole substrate is constrained by the HL2000

Figure 23 Explosion drawing of the LTCC flow sensor

Figure 23 illustrates the final sensor design [Loh12] The bottom part of the channel was filled with sacrificial material which was overprinted by glass and platinum paste to create the heater structure

Afterwards the upper channel part was laminated on top of it so that the heater was centered With this tech-nique a lot of different heater geometries can be real-ized

Table 4 Sensor characteristics dH ndash heater thickness LH ndash heater length RH0 ndash heater resistance (T = 25 degC) RH100 ndash heater resistance (T = 100 degC)

Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31

The prototypes show the expected relation between the current through the heating wire and the fluid flow Figure 24 shows the characteristic curve of the sensor In future work a measuring circuit will be integrated in

the system to complete the sensor and to linearize the current-flow correlation

Figure 24 LTCC flow sensor and characteristic curve heater current (I) vs fluid flow (v)

6 Conclusions

The selected examples show that LTCC is a well suited integration platform for mechanical sensors Their de-sign and fabrication requires a deep understanding and control of material issues (eg material interactions during co-firing shrinkage behavior sacrificial layer) Besides this it is necessary to expand the LTCC process flow on the generation of the required 3D structural el-ements

Pressure force acceleration and flow sensors were de-veloped following a defined design and manufacturing flow

It was shown that the sensors have an excellent and stable functionality in all cases The main advantage of the presented sensors is the quasi-monolithic design It prevents thermo-mechanical strain which normally oc-curs when parts of different materials are bonded


270

Using the demonstrated material and process expan-sions and improvements LTCC-based sensors for me-chanical quantities are interesting and cost-effective alternatives to existing technical solutions

References

1 Jones W K Liu Y Larsen B Wang P and Zampi-no M ldquoChemical Structural and Mechanical Properties of the LTCC Tapesldquo The International Journal of Microcircuits and Electronic Packaging Volume 23 Number 4 Fourth Quarter 2000 S 469

2 Maeder T Jacq C and Ryser P ldquoLong-term me-chanical reliability of ceramic thick-film circuits and mechanical sensors under static loadrdquo Sen-sors and Actuators A in press (2012)

3 Duan K Mai YW and Cotterell B ldquoCyclic fatigue of a sintered Al2O3 ZrO2 ceramicrdquo J Mater Sci 30 [20] 5192-5198 (1995)

4 Partsch U ldquoUntersuchungen zum Schwindungs-verhalten von LTCC-Multilayernrdquo Diploma Thesis 1996 Friedrich-Schiller-Universitaumlt Jena

5 Eberstein M Rabe T and Schiller W A bdquoInflu-ences of the Glass Phase on Densification Micro-structure and Properties of Low-Temperature Co-Fired Ceramics Int J Appl Ceram Technol 3 [6] 428ndash436 (2006)

6 Jau-Ho J and Chia-Ruey C ldquoInterfacial Reaction Kinetics between Silver and Ceramic-Filled Glass Substraterdquo J Am Ceram Soc 87 [7] 1287ndash1293 (2004)

7 Rabe T Glitzky C Naghib-Zadeh H Oder G Eberstein M and Toumlpfer J ldquoSilver in LTCC ndash Inter-facial Reactions Transport Processes and Influ-ence on Properties of Ceramics CICMT ndash Ceramic Interconnect and Ceramic Microsystems Technol-ogy Denver 2009 Proceedings

8 Lenz C Ziesche S Partsch U Neubert H ldquoLow Temperature Cofired Ceramics (LTCC)-Based Miniaturized Load Cellsrdquo Proc 35th International Spring Seminar on Electronics Technology ISSE Bad Aussee (2012)

9 Pike G E and Seager C H Electrical Properties and Conduction Mechanisms of Ru-Based Thick Film (Cermet) Resistors Journal of Applied Phys-ics 1277 S 5152

10 Vionnet-Menot S Grimaldi C Maeder T Ryser P and Straumlssler S bdquoTunneling-percolation origin of nonuniversality Theory and experimentsldquo Phys RevB 71 [6] 064201 (2005)

11 Prudenziati M (Ed) ldquoHandbook of Sensors and Actuators 1 Thick Film Sensorsrdquo Elsevier Science BV 1994

12 Dietrich S Kretzschmar C Partsch C Reben-klau L bdquoReliability and Effective Signal-to-Noise Ratio of Ru02-based Thick Film Strain Gauges The Effect of Conductive and Glass Particle Sizeldquo Elec-tronics Technology 2009 ISSE2009 32nd Interna-tional Spring Seminar

13 Maeder T Jacq C Fournier Y Ryser P ldquoFormu-lation and processing of screen-printing vehicles for sacrificial layers on thick-film and LTCC sub-stratesrdquo 32nd IMAPS ndash International Microelec-tronics and Packaging Pultusk (2008) Proceed-ings

14 Maeder T Jacq C Fournier Y Hraiz W Ryser P bdquoStructuration of zero-shrinkage LTCC using min-eral sacrificial materialsrdquo 17th EMPC ndash European Microelectronics and Packaging Conference amp Ex-hibition Rimini (2009) Proceedings

15 Birol H Maeder T Ryser P ldquoApplication of graph-ite-based sacrificial layers for fabrication of LTCC (low temperature co-fired ceramic) membranes and micro-channelsrdquo J Micromech Microeng 17 p 50-60 (2007)

16 Malecha K Maeder T Jacq C ldquoFabrication of membranes and microchannels in low-tempera-ture co-fired ceramic (LTCC) substrate using novel water-based sacrificial carbon pastesrdquo Journal of the European Ceramic Society 32 (12) 3277-3286 2012

17 Lohrberg C rdquoEntwicklung LTCC-basierter hoch-empfindlicher Stroumlmungssensorenrdquo Diploma the-sis 2012 Dresden University of Technology

18 Rabe T Schiller W A bdquoZero Shrinkage of LTCC by Self-Constrained Sinteringrdquo Int J Appl Ceram Technol 2 [5] p 374-382 (2005)

19 Wagner M bdquoProzessparameter und ihr Einfluss auf die Schwindungsgenauigkeit von hochintegrierten keramischen Mehrlagenschal-tungenldquo Diss Thesis of University Erlangen-Nuremberg

20 Neubert H Partsch U Fleischer D Gruchow M Kamusella A Pham T ldquoThick Film Accelerom-eters in LTCC-Technology ndash Design Optimization Fabrication and Characterizationrdquo IMAPS Journal of Microelectronics and Electronic Packaging 5 (2008) 4 150-155

21 Neubert H Uncertainty-Based Design Optimi-zation of MEMSNEMS In Gerald Gerlach Klaus Wolter (Eds) Bio and Nano Packaging Techniques for Electron Devices - Advances in Electronic De-vice Packaging 123 Springer 2012 pp 119-140

22 Slosarcik S Bansky J Dovica M Tkac M Un-conventional Application of Low Temperature Cofired Ceramics for (Under-) Pressure Sensors J Electrical Engineering (48) 1997 Nr 9-10

23 Partsch U Gebhardt S Arndt D Georgi H Neubert H Fleischer D and Gruchow M bdquoLTCC-


271

Based Sensors for Mechanical Quantitiesrdquo CICMT ndash Ceramic Interconnect and Ceramic Microsys-tems Technology Denver 2009 Proceedings

24 Fournier Y Maeder Th Boutinard-Rouelle G Barras A Craquelin N and Ryser P ldquoIntegrated LTCC Pressure Flow Temperature Multisensor for Compressed Air Diagnosticsrdquo Sensors 2010 10(12) 11156-11173

25 Thelemann T Die LTCC-Technologie als Basis von sensorischen aktorischen und fluidischen Kom-ponenten fuumlr Mikrosysteme Diss thesis of Ilme-nau University of Technology 2005

Arrived 30 08 2012Accepted 20 11 2012


261


Table 1 Mechanical properties of different ceramics (CTE hellip coefficient of thermal expansion (20 400degC) E hellipYoungrsquos modulus sB hellip bending strength)

Material CTE E sB sBE

ppm K GPa MPa 10-3

96 Al2O3 1) 76 350 310 09

998 Al2O3 2) 75 406 630 16

LTCC 3) 58 120 320 27YSZ 4) 112 210 1050 50ZTA 5) 81 357 1350 38

Datasheet values 1) CeramTec V38 2) CeramTec RK 87 3)

Du Pont DP 951 4) CeramTec MZ 429 5) CeramTec DC 25

The sBE ratio determines the dimension of the deform-ing bodies in terms of sensitivity and overload stabil-ity The larger the sBE ratio the smaller the deforming bodies can be designed It can be seen that LTCC is well suited because of a sBE ratio of 27 However YSZ (sBE = 5) and ZTA (sBE = 38) have a better mechanical per-formance but embedding eg of low sintering noble metals as well as resistors can only be realized using a low temperature firing system

Different types of LTCC show a different bending strength behavior [1] Best values can be obtained us-ing the Du Pontrsquos Green Tape 951 system (Figure 1) It must however be noted that in order to ensure long-term reliability static [2] and cyclic fatigue [3] must also be accounted for

Figure 1 Comparison of different LTCC types regard-ing their fractural strength (re-calculated after [1])

The 951 system offers different tape thicknesses (50 ndash 254 microm unfired) as well as a full system of the required pastes (innerouter conductors via outerinner resis-tors) This makes it particularly well suited for the fab-rication of sensors

The shrinkage control of LTCC multilayer components plays an important role regarding the final dimension-al control eg for advanced electronic packages (chip sized packages flip chip) but also in the case of me-chanical sensors

Shrinkage occurring during firing can be influenced by controlling the process factors (of lamination and fir-ing) Figure 2 shows the influence of lamination tem-perature and pressure

Figure 2 LTCC DP 951 X-Y shrinkage (SX-Y) vs lamina-tion temperature (T) and pressure (p) [4]

22 Functional Thick Films

221 Metallization

In many cases LTCC-suited inner metallization pastes are silver-based The interaction between silver and LTCC during co-firing was described in the past by sev-eral authors eg [4-6] it can lead to significant warping of thin and 3D structured LTCC geometries

Figure 4 LTCC DP 951 X-Y shrinkage (SX-Y) vs heat up ramp (HR) and metallization degree (MD) [4]

σ

262



222 Piezo-Resistors







(1)





Piezo-resistor

Wheat-stone

bridge

Deform-ablebody

Force F Stressσ

Strainε

ΔR (ε)

Bridge voltage UB

Displacementw




263













264





















265






4 Sensor design









5 Applications

51 Pressure Sensor




266












[FS] [FS]

45 220 50 04 006 00545 150 15 11 004 00145 45 02 46 007 008



52 Force Sensor



267








Nominal load

F N

FB

[N]S

[mV( VN)]L

[FS]TC-S

[SK]

2 N 4 26 lt 06 0025 N gt 10 06 lt 04 003

10 N gt 20 01 lt 10 002






∆


268










54 Flow Sensor


∆


269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






262



222 Piezo-Resistors







(1)





Piezo-resistor

Wheat-stone

bridge

Deform-ablebody

Force F Stressσ

Strainε

ΔR (ε)

Bridge voltage UB

Displacementw




263













264





















265






4 Sensor design









5 Applications

51 Pressure Sensor




266












[FS] [FS]

45 220 50 04 006 00545 150 15 11 004 00145 45 02 46 007 008



52 Force Sensor



267








Nominal load

F N

FB

[N]S

[mV( VN)]L

[FS]TC-S

[SK]

2 N 4 26 lt 06 0025 N gt 10 06 lt 04 003

10 N gt 20 01 lt 10 002






∆


268










54 Flow Sensor


∆


269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






263













264





















265






4 Sensor design









5 Applications

51 Pressure Sensor




266












[FS] [FS]

45 220 50 04 006 00545 150 15 11 004 00145 45 02 46 007 008



52 Force Sensor



267








Nominal load

F N

FB

[N]S

[mV( VN)]L

[FS]TC-S

[SK]

2 N 4 26 lt 06 0025 N gt 10 06 lt 04 003

10 N gt 20 01 lt 10 002






∆


268










54 Flow Sensor


∆


269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






264





















265






4 Sensor design









5 Applications

51 Pressure Sensor




266












[FS] [FS]

45 220 50 04 006 00545 150 15 11 004 00145 45 02 46 007 008



52 Force Sensor



267








Nominal load

F N

FB

[N]S

[mV( VN)]L

[FS]TC-S

[SK]

2 N 4 26 lt 06 0025 N gt 10 06 lt 04 003

10 N gt 20 01 lt 10 002






∆


268










54 Flow Sensor


∆


269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






265






4 Sensor design









5 Applications

51 Pressure Sensor




266












[FS] [FS]

45 220 50 04 006 00545 150 15 11 004 00145 45 02 46 007 008



52 Force Sensor



267








Nominal load

F N

FB

[N]S

[mV( VN)]L

[FS]TC-S

[SK]

2 N 4 26 lt 06 0025 N gt 10 06 lt 04 003

10 N gt 20 01 lt 10 002






∆


268










54 Flow Sensor


∆


269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






266












[FS] [FS]

45 220 50 04 006 00545 150 15 11 004 00145 45 02 46 007 008



52 Force Sensor



267








Nominal load

F N

FB

[N]S

[mV( VN)]L

[FS]TC-S

[SK]

2 N 4 26 lt 06 0025 N gt 10 06 lt 04 003

10 N gt 20 01 lt 10 002






∆


268










54 Flow Sensor


∆


269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






267








Nominal load

F N

FB

[N]S

[mV( VN)]L

[FS]TC-S

[SK]

2 N 4 26 lt 06 0025 N gt 10 06 lt 04 003

10 N gt 20 01 lt 10 002






∆


268










54 Flow Sensor


∆


269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






268










54 Flow Sensor


∆


269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






269









Channel cross

sectionA C

dH[microm]

LH[mm]

RH0[Ω]

RH100[Ω]

08 mm 50 4 25 31




6 Conclusions





270


References

























271






270


References

























271






271






ltcc-based sensors for mechanical quantities papers/midem_42(2012)4p260.pdf · 262 figure 4 shows...

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